In the horse industry development of an accurate, economical method for the precise control of ovulation in the mare would greatly benefit reproductive management of mares and stallions. The mares' extended estrus period, with ovulation at any time from 1 to 10 days after the beginning of estrus, has made reproductive management of mares time-consuming, expensive and most importantly, inefficient. In the mare, GnRH or its analogs are beginning to be used as alternative non-antigenic substitutes to replace hCG to hasten ovulation in preovulatory mares. This is because repeated use of hCG has been associated with decreased response [Sullivan, J., J Am. Vet. Med. Assoc. 63:895(1973)] and anti-hCG antibody formation [Roser, J., J. Reprod. Fert. Suppl. 173-179(1974)]. Current data suggest that ovulation induction with potent GnRH analogs requires multiple injections of very low doses [Harrison, L., et al., J. Eq. Vet Sci. 11:163-166(1991)] or a very high dose given as a slow releasing implant [Jochle, W. et al., J. Eq. Vet. Sci. 44:632(1994)].
The selection of an appropriate drug delivery system should be based on the pharmacokinetic and pharmacodynamic properties of the drug. The importance of the pharmacodynamic properties of a drug is especially relevant in the case of hormones that target specific high affinity receptors to produce their effect. In the case of GnRH this relationship is dependent on multiple elements including species, reproductive status and complex concentration/presentation effects of the peptide and pituitary responsiveness to it.
Applicants have discovered that certain compositions are suitable for controlled release of GnRH analogs, particularly for the purpose of advancing ovulation in mares. The composition includes a system based on sucrose acetate isobutyrate (SAIB) a fully-esterified sucrose molecule. SAIB is a low molecular wt material that has many of properties associated with polymeric materials. Because SAIB is a non-polymer, dilution with only small amounts of solvents are required to give an easily-injectable solution.